BACKGROUND OF THE INVENTION
[0001] The present invention relates to orifice assemblies for fluid flow conduits and more
particularly to orifice assemblies for increasing the resistance to fluid flowing
through such conduits. The present invention has particular application in forced
circulation steam generating units. In steam generating units constructed to operate
with forced circulation, a multiplicity of steam generating tubes are connected to
a common header from which they receive their supply of water thereby forming a multiplicity
of parallel tube circuits in which steam is generated. The common header is connected
to the discharge of a pump which receives its water from a steam and water separating
drum. The tube circuits discharge the steam and water mixture into the steam and water
separating drum.
[0002] In the past, orifices or other flow restricting means have been employed at the entrance
of each of the multiplicity of tubes for controlling the distribution of water from
the common header to the individual tube circuits. A high flow resistance is required
to assure uniform flow distribution and prevention of flow reversals in shaded water
wall panels and flow starvation of adjacent tubes in the event of a single tube rupture.
In connection with the latter aspect of flow starvation, the orifices serve to throttle
or choke the flow to the ruptured steam generating tube. This then insures that the
remaining mass flow from the common header will be distributed to the remaining, nonruptured
steam generating tubes.
[0003] Present day means of securing the orifices into the tubes are as follows: First a
ring is welded into the inner surface of each tube. Then a plate having a suitably
sized orifice therein is mechanically clamped to the ring. This type of fastening
has the advantage of being able to replace the orifice plate at a later date if it
is determined that the wrong sized orifice has been placed in some of the tubes, or
in the event the orifice plate becomes corroded to the point of being inoperative.
This type of securing orifices in tubes also has some disadvantages. Because of the
requirement of welding in the ring, it is an expensive procedure. Also because the
ring and its associated orifice plate is located in a horizontal component of the
tubes in many instances, it is not possible to completely drain the tubes during a
shutdown period, or when the tubes are given an acid-wash. Also in high pressure steam
generators, such as one operating at supercritical pressure, there can be considerable
leakage of fluid between the orifice plate and the welded-in ring, making the orifice
ineffective in accurately throttling the flow a desired amount.
SUMMARY OF THE INVENTION
[0004] According to the present invention, orifices are explosively expanded into the inlets
of steam generating tubes, so as to form a.fluid tight seal between the cup containing
the orifice therein and the inner walls of the tube. Also, the orifice can be positioned
at the bottom of the tube so that even when the orifice is located in a horizontal
component of a tube it permits the tube to be completely self-drainable during shutdown
or after an acid-washing of the tubes. By means of a heat shrinking procedure, the
orifice can be easily removed from the tube if this becomes necessary.
BRIEF DESCRIPTION OF THE DRAWINGS
[0005]
Figure 1 is a schematic representation of a forced circulation steam generating unit
in which the present invention is incorporated.
Figure 2 is an enlarged partial sectional side view of a header showing one tube connection,
and the manner in which its associated orifice assembly is secured in place;
Figure 3 is a view similar to Figure 2 showing an alternative orifice assembly; and
Figure 4 is a view taken on line 4-4 of Figure 3.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0006] Referring now to Figure 1, numeral 10 depicts a steam generating unit in its entirety.
The unit has a furnace 12 into which fuel and air are introduced through burners (not
shown). The hot combustion gases flow upwardly within the furnace 12 then downwardly
through rear path 14 giving up heat along the way to the fluid passing through the
various heat exchangers positioned therein, before being exhausted to the atmosphere
through a stack connected to duct 16.
[0007] Water flows into and through an economizer 18 located in the rear path, then into
the drum 20 down through pipe 22 to the pump 24. Water from the pump outlet flows
to distribution headers 26, which supply the tubes 28, which are welded together to
form the walls of the furnace chamber. The steam- water mixture leaving tubes 28 flows
into the drum 20 with the water being separated therein and again flowing to the pump
24. The steam passes through superheaters 30, 32 before flowing to a steam turbine
(not shown). The inlets of all the tubes 28 which line the furnace walls each contain
an orifice assembly 36 (Fig. 2) to assure uniform flow of fluid to each of the tubes.
[0008] Looking now to Figure 2, the details of one orifice assembly, and the manner in which
it is securely installed, is shown. The orifice assembly 36 contains an orifice 38
located in an end wall 40. The opposite end is open and has a lip or flange 42 to
initially accurate position the orifice plug during assembly. The assembly is of such
outer diameter that it can be easily slid into the tube 28 from inside of header 26.
Header 26 will either be of suitable size to permit workmen to climb inside, or it
will be provided with handholes permitting ready access to the tube inlets. A polyethylene
tube 44 is positioned inside the orifice assembly, which has a core within which is
positioned an explosive 46. The polyethylene tube has a closed end 48 so that the
residue from the explosion does not contaminate the interior of tube 28, and also
to prevent distortion of the orifice 38. A flange 50 on the polyethylene tube 44 accurately
positions it within the orifice assembly 36.
[0009] Any suitable detonating means can be used for detonating the charge and more than
one explosive can be detonated at the same time. The explosive can be any of several
suitable for this purpose. One such explosive is a Primacord fuse, preferably of PETN
(pentaerythritol tetranitrate). Upon detonation, the polyethylene tube momentarily
expands causing the orifice assembly 36 to expand into tight engagement with the inner
wall of tube 28. The mechanical bond formed between the orifice 36 and the tube 28
is such that it prevents fluid leakage therebetween, even at extremely high pressures
and temperatures. After an orifice assembly has been secured in the inlet of each
tube 28 and the plastic tubes 44 removed, the header 26 can be closed and the unit
will then be ready to be put into operation.
[0010] Looking now to Figs. 3 and 4, an alternative form of the invention is shown. As mentioned
earlier in the specification, it is desirable to be able to completely drain the boiler
tubes 28 even when the orifice is contained in a . part of the tube having a horizontal
component. By making the opening 60 eccentric and by locating it in the very bottom
of the tube 28, this is made possible. This is easily accomplished when the orifice
assemblies are explosively secured in place in accordance with the invention.
[0011] If it becomes necessary to replace any of the orifice assemblies at a later time,
such.can be accomplished by inserting a heating element into the orifice assembly,
and heating it to a temperature above the plastic deformation temperature of the assembly
material. Upon cooling the assembly will shrink to less than its original size, allowing
easy removal thereof. This process is described in U.S. Application Serial Number
584,703 filed on February 29, 1984.
1. In a steam generator having a furnace with tubes lining the walls of the furnace,
and header means for supplying fluid to the tubes, and restriction means positioned
in the inlet of each tube for insuring adequate flow of fluid to each to said tubes,
characterized by the method of forming the restriction means in each tube including
the step of positioning a hollow plug in each tube, which plug is opened at one end
and has a restricted opening in the other end and explosively expanding each plug
into tight engagement with the inner wall of each tube.
2. The method set forth in Claim 1, further characterized in that the hollow plugs
are located in a portion of each tube having a horizontal component, and the restricted
opening in each plug is positioned as to be at the bottom of each tube.
3.. In combination, a steam generator having a furnace, tubes lining the walls of
the furnace, header means for supplying fluid to the tubes, pump means for forcing
fluid through the header means into the tubes, restriction means positioned in the
inlet of each tube for insuring adequate flow of fluid to each of the tubes, characterized
in that each of the restriction means includes a hollow plug, open at one end and
having a restricted opening in its other end, positioned at the bottom of each tube,
each plug being explosively expanded into a portion of its associated tube which has
a horizontal component.